updated bootstrap to pin percona-toolkit version

previously it was using a URL to download an RPM
which was in the LATEST directory on the percona server
which means the specific version there would change

by using a specific version we'll have to update this
occassionally but it's more reliable for now
updated query explain code to check for array indexes (fixes #75)

there was some work done on this in a previous commit, but this
update fixes another minor issue which was preventing the correct
max column width from being used.